1. Field of the Invention
The present invention relates to an image forming apparatus for forming an image on a recording medium.
2. Description of the Related Art
In general, an ink jet recording apparatus as one type of image forming apparatus has a recording head which discharges ink droplets to form and record on a recording medium characters, images and so forth (collectively referred to as an xe2x80x9cimagexe2x80x9d hereinafter). The term xe2x80x9crecording mediumxe2x80x9d is used in this specification to generally encompass web or sheet-type media which have small thicknesses and on which images are formable by an ink. Thus, the recording medium may be a sheet of plain paper, a sheet of resinous recording medium as a substitute for the plain paper, a postcard, an envelope, a label, and so on. In this specification, the term xe2x80x9csheetsxe2x80x9d is used to encompass all such possible recording media.
Ink jet recording apparatuses offer various advantages such as ease of miniaturization of recording heads, capability of recording fine images at high speed, low running cost, reduced noise level due to non-impact nature, and ease of recording color images with multiple color inks.
In general, an ink jet recording apparatus is configured such that a recording medium sheet is fed from a sheet feed section onto a platen which opposes a recording head and which supports the recording medium sheet at the reverse side thereof while the recording head records an image on the obverse side of the recording medium sheet. The recording medium sheet carrying the image recorded thereon is ejected from the apparatus through an ejecting section which is disposed at the downstream end as viewed in the direction of convey of the recording medium sheet.
To be more specific, reference is made to FIG. 8 which shows an overall arrangement of a known ink jet recording apparatus generally denoted by 1, FIG. 9 which is a sectional view of the ink jet recording apparatus 1, and FIG. 10 which shows a portion of the apparatus 1 including a recording medium sheet ejecting section denoted by 4. In FIG. 10, components such as a conveyor roller 36, a pinch roller 37, spurs 42, an ejecting roller 41, as well as a sheet P, are schematically shown in a perspective view. Since FIG. 10 is a schematic illustration, the dimensional relationships between the components in this Figure do not exactly conform with those in FIG. 9. Reference is made also to FIG. 11 which is a sectional view taken along the line Cxe2x80x94C of FIG. 10.
Referring to FIGS. 8 to 11, a stack of recording medium sheets P is placed on a sheet feed section 2, and the recording medium sheets are fed by a sheet feed roller 22 in one-by-one fashion starting from the topmost sheet toward a position where a recording head 7 is located. The sheet P is nipped between the pinch roller 37 and the conveyor roller 36 which is driven by power derived from a drive source which is not shown, and is conveyed to a recording start position on a platen 34.
The recording head 7 is detachably secured to a carriage 50 which is carried by a guide shaft 81 for reciprocal movement along the guide shaft 81 in directions which cross the direction of convey of the sheet P. Thus, the recording head 7 performs recording of images on the sheet P which is being conveyed, while moving in directions that cross the direction of movement of the sheet P.
The ejection roller 41 is driven by the conveyor roller 36 through a transmission roller 40. The ejection roller 41 has a plurality of roller elements 41a which are made of an elastomer or a rubber and which are arranged in the direction of width of the sheet P so as to convey the sheet P.
There are a plurality of spurs 42 each of which is held in pressure contact with the associated roller element 41a. Each spur 42 rotates in contact with the recording medium sheet which carries an image formed thereon. In order to prevent transfer of the ink, therefore, the spur 42 has a plurality of radial projections with pointed ends.
The recording sheet medium P is pinched between and conveyed by the conveyor roller 36 and the pinch roller 37, until its leading end is caught between the ejection roller 41 and the spur 42. The sheet P, after its trailing end has cleared the nip between the conveyor roller 36 and the pinch roller 37, is conveyed by the ejection roller 41 and the spur 42. The sheet P is then ejected onto and received by a sheet ejection tray 100.
The upper surface of the platen 34 has a plurality of ribs 34a which project therefrom and which extend in the direction of movement of the sheet P. The ribs are arranged at intervals in a direction which crosses the direction of movement of the sheet P. Each rib 34a is aligned with one of the spurs 42 which is disposed downstream of the rib 34a, as viewed in the direction of movement of the sheet P. In other words, each rib 34 and the associated spur 42 are disposed on a common straight line which extends in the direction of movement of the sheet P. With this arrangement, crests of waviness of the recording medium sheet, which occur when the sheet P is wetted by the ink, are flattened by the effect of the spurs 42. This waviness will be referred to as xe2x80x9ccocklingxe2x80x9d, hereinafter. In addition, the sheet P is caused to cockle downward at portions thereof not supported by the ribs, so that floating of the sheet P is minimized while the sheet passes through the recording section.
The cockling of the sheet P occurs at regular portions thereof due to the regular arrangement of the ribs 34a. This means that the distance between the sheet P and the recording head 7 regularly varies along the breadth of the sheet P, which if not corrected may degrade the quality of the recorded image. In order to obviate this problem, the discharge of the ink from the recording head 7 is performed so as to compensate for any influence caused by the presence of the ribs the height of which is known, whereby any degradation of the image quality which otherwise may be caused by the presence of the ribs is effectively corrected.
The problem of cockling is serious particularly when an image is xe2x80x9cpaintedxe2x80x9d with a dye color ink which is permeable to the material of the recording medium sheet. In order that images of high quality are stably formed with this type of ink, it is necessary to strictly maintain a constant gap between the sheet P and the recording head, thereby minimizing undesirable effects of cockling.
Image recording apparatuses also have recently been proposed which use pigment-type black ink in order to increase the density of black color of images. A considerably long time is required for this type of ink to be dried and fixed after being deposited on a recording medium sheet. A problem is therefore encountered with the use of this type of ink. It is assumed here that two recording medium sheets, a first sheet and a second sheet, are successively ejected. In such a case, the leading end of the second sheet rubs the ink which is still wet on the first sheet which has been ejected. Consequently, the image on the first recording medium sheet is fouled. The phenomenon of fouling the image on the leading recording medium sheet is generally referred to as xe2x80x9csmearxe2x80x9d.
The image forming apparatus shown in FIG. 9 has sheet supporting members 47 which are provided in the sheet ejection section 4 and which serve as means for preventing smearing. The sheet supporting members 47 serve to support the sheet P after recording so as to prolong the time required for the sheet P to land on a sheet ejection tray 200 after the discharged ink is deposited on the sheet P. A subsequent sheet P, i.e., the second sheet, is then ejected to fall onto the recording medium sheet when the ink on the preceding recording sheet medium P, i.e., the first sheet, has been dried, thus preventing smearing of the image on the first sheet.
Hitherto, the sheet supporting members 47 are arranged in the direction of breadth of the sheet P, i.e., in the direction which crosses the direction of movement of the sheet P. The supporting projections 47 are projectable in the downstream direction from the platen 34 and retractable into the platen 34.
The sheet supporting members 47, when projected in the downstream direction, guide the sheet P to a level higher than the horizontal plane of the sheet P in the recording section, so as to support the sheet P by making use of stiffness inherent to the sheet P, whereby the time required for the sheet P to land on the sheet ejection tray 200 after the image formation is prolonged.
In the known arrangement, the sheet supporting members 47 are positioned to intervene between the adjacent spurs 42 such that the projections 47 and the spur 42 are disposed alternately in the breadthwise direction of the sheet. The sheet supporting members thus arranged tend to lift the portions of the sheet P that have cockled to be convex downward in the recording section, thus canceling the cockle of the sheet P. Rather, these sheet supporting members 47 cause the sheet P to cockle and project upward at portions between adjacent spurs 42, which in turn causes these portions of the sheet P to float significantly also in the recording section. This poses a risk for the sheet P to undesirably contact the recording head 7 or the carriage 50, resulting in fouling of the recording surface due to rubbing and, in the worst case, damaging of the recording head 7 particularly at the ink jet surface thereof.
The sheet supporting members 47 are arranged to project from the platen 34 when the leading end of the sheet P which is undergoing the image forming process has cleared the spurs 42. Therefore, the free ends of the sheet supporting members 47 strongly impinge against the sheet P, which hampers the image forming operation to impair the quality of the image.
Another problem is that the use of discrete sheet supporting members 47 arranged in the direction of breadth of the sheet P undesirably raises the production cost of the image forming apparatus.
Accordingly, it is an object of the present invention to provide an image forming apparatus which obviates problems such as impairment of image quality due to smear, while suppressing floating of the recording medium sheet and avoiding rubbing of the recording medium sheet against the recording head.
To this end, according to the present invention, there is provided an image forming apparatus for forming an image on a recording medium by discharging an ink from a recording head, comprising: at least one ejection roller for causing the recording medium to move at a position downstream of the recording head as viewed in the direction of movement of the recording medium; at least one idle roller driven by the ejection roller; and at least one recording medium supporting member disposed downstream of the ejection roller as viewed in the direction of movement of the recording medium and arranged to guide the recording medium, the recording medium supporting member and the idle roller being located on a common straight line which extends in the direction of movement of the recording medium.